I use a kriega 10L. I really like thier stuff and it's modular so you can add on and build a nice set. I just got the tank mount for my 10L for when I'm using a top case.
Here is my tank bag/tail bag. I can switch in under a minute. The kriega bags can also link to one another for long trips. I'm waiting on panniers and a top case but will still use the bag on the tail for short trips to work.